How Heat Makes Your Food Change

FOOD

Q. The longer I cook my egg, the harder it gets. The longer I cook my potato, the softer it gets. Why does heat have such different effects on foods?

A. The short answer is that cooking makes proteins harder and carbohydrates softer.

Eggs are quite unusual in their composition, as befits their unique function in life. The dried contents of an egg are just about half protein and half fat. The yolks are mostly (70 percent) fat while the whites are mostly (85 percent) protein. You know that heat doesn't affect the texture of fat very much, so we'll concentrate on what happens to the protein in the egg white. And you also know that we're not going to get out of this without looking at what the molecules are doing, right?

The albumins in albumen -- no kidding; egg whites are called albumen, with an "e," but they contain proteins called albumins, with an "i" -- are made up of long, stringy molecules, coiled up into globs like very loose balls of yarn. When heated, these balls partially unravel and then stick together here and there, making an unholy tangle like a can of spot-welded worms. Now when the molecules of a substance change from a bunch of loose balls into a welded-together jumble, the stuff is obviously going to lose its fluidity. So liquid egg albumen, when heated above approximately 150 degrees Fahrenheit, coagulates into a firm, white, opaque gel.

The hotter and longer you heat them, the more the protein molecules will spot-weld to one another. So the more you cook an egg, the firmer its white will become, ranging from the glop of soft-boiled to the rubber of hard-boiled to the leather of an "over, well" hash-house special.

Meanwhile, the protein in the egg's yolk coagulates in much the same way, but not until it reaches a somewhat-higher temperature. Also, its abundant fat acts as a lubricant between the protein molecules, so they can't weld together as much. That's why the yolk doesn't get as tough as the white.

We're leaving meats out of this discussion, because the toughness or tenderness of cooked meat isn't a simple matter of protein coagulation; it depends in a very complex way on how the protein fibers are put together in the meat's structure and on how it is being cooked. For example, during prolonged cooking, meat can get tenderer at first and then tougher later on.

Now about that potato and other foods that contain a lot of carbohydrates. Starches and sugars cook easily; they even dissolve in hot water to speed the process. When you bake a potato, some of the starch actually dissolves in the steam.

But there's one very tough and insoluble carbohydrate that is present in all of our fruits and vegetables: cellulose. The cell walls of plants are made of cellulose fibers held together by a cement of pectin and other water-soluble carbohydrates. This structure is what makes vegetables such as cabbage, carrots and celery -- and potatoes -- so firm and crisp. But put the heat on these tough guys and the pectin cement dissolves out into the liquids released by the heat, and the rigid cellulose structure is severely weakened. That's why cooked vegetables are softer than raw vegetables.